1. Field of the Invention
This invention relates to an intake system for an engine provided with a turbocharger.
2. Description of the Prior Art
There has been a great demand for increase in the engine output torque of a turbocharged engine in the low engine speed range. That is, since the exhaust pressure is low in the low engine speed range, the engine cannot be satisfactorily supercharged by the turbocharger. Further, as is well known, in the turbocharged engine, combustion energy can be better used and fuel economy can be improved over the entire operating range of the engine by increasing the geometric compression ratio to at least 9, preferably to at least 10. However, when the geometric compression ratio is increased to such an extent, the effective compression ratio is increased and the temperature of the air-fuel mixture is increased to generate knocking of the engine. Although the occurrence of engine knock can be suppressed by retarding the intake valve closing timing, thereby reducing the effective compression ratio, if the intake valve closing timing is simply retarded, the air-fuel mixture will be blown back into the intake passage in the low engine speed range. This, coupled with the fact that the charging effect of the turbocharger is poor in the low engine speed range, further reduces the engine output torque in the low engine speed range.
In order to compensate for the reduction in the engine output torque in the low engine speed range by supercharging the engine by a kinetic effect of intake air, the intake system must be arranged so that the engine can be supercharged by the kinetic effect of intake air at a substantially low engine speed. However, when the intake system is so arranged, the intake system will act to reduce the intake air charging efficiency at the high engine speed range and the engine output torque will be lowered in spite of supercharging by the turbocharger.